Incorporation of α-MnO2 Nanoflowers into Zinc-Terephthalate Metal–Organic Frameworks for High-Performance Asymmetric Supercapacitors

Herein, we report the synthesis of α-MnO2 nanoflower-incorporated zinc-terephthalate MOFs (MnO2@Zn-MOFs) via the conventional solution phase synthesis technique as an electrode material for supercapacitor applications. The material was characterized by powder-X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy techniques. The prepared electrode material exhibited a specific capacitance of 880.58 F g–1 at 5 A g–1, which is higher than the pure Zn-BDC (610.83 F g–1) and pure α-MnO2 (541.69 F g–1). Also, it showed a 94% capacitance retention of its initial value after 10,000 cycles at 10 A g–1. The improved performance is attributed to the increased number of reactive sites and improved redox activity due to MnO2 inclusion. Moreover, an asymmetric supercapacitor assembled using MnO2@Zn-MOF as the anode and carbon black as the cathode delivered a specific capacitance of 160 F g–1 at 3 A g–1 with a high energy density of 40.68 W h kg–1 at a power density of 20.24 kW kg–1 with an operating potential of 0–1.35 V. The ASC also exhibited a good cycle stability of 90% of its initial capacitance.


INTRODUCTION
In the present century, the exponentially increasing global population and the tumbling non-renewable resources pushed us to utilize consumable renewable energy sources. Though the production of energy from reliable sources can be made possible, the search for proper energy storage systems remains a challenge. 1 In the midst of traditionally available energy storage systems, supercapacitors sprang up as an alternative to batteries, considering the poor life cycle and feeble power density of batteries. 2 Despite the fact that batteries have their unique applications, certain areas with the requirement of high-power density and longer life cycles make them inutile. On the other hand, supercapacitors can provide high power and undergo longer charging and discharging cycles. They have been successfully employed in various applications as multiresponsive healable supercapacitors, 3 piezoelectric-driven self-charging supercapacitors, 4 and in wearable and portable electronic devices by assembling the supercapacitor as a device with flexible electrodes. 5,6 Researchers are currently focused on designing electrode materials with different morphologies and larger surface areas to increase their electrochemical performance. 7,8 However, factors like poor ion diffusion and inadequate structural stability prevent them from being used in certain fields that need high rate and durable energy conversion and storage. 9 This drawback of poor energy density can be overcome by selecting and properly optimizing electrode materials of the supercapacitors. 10,11 Lately, MOFs and their derivatives were introduced into the field of energy storage technologies due to their excellent porosity, tunable structure, and stable morphology. 12 One of the special characteristics of MOFs is their coordinative arrangement and this kind of arrangement is very much suitable for charge transfer and storage; therefore, it can be used as electrode materials in electrochemical supercapacitors. 13,14 However, most of the MOFs show poor electrical conductivity. 15 Pristine MOFs in the application of energy storage are typically viewed as insufficient to produce the desired outcomes due to their relatively low intrinsic conductivity. 16 The electrical conductivity and electrochemical reaction sites of pristine MOFs could be improved by incorporating conductive additives [e.g.,: carbon black (CB), conductive polymers, and so forth.] and metal oxides. 17 Moreover, the pristine MOF can be used as a sacrificial template to form a pure metal oxide, metal oxide/carbon composite, or carbon, which can be used as electrode materials for supercapacitors. 18,19 Among the choice of metal oxide additives, MnO 2 is a promising material due to its abundant availability, lack of toxicity, low cost, several oxidation states, wide voltage window, high surface area, and high theoretical capacitance. 20 Additionally, MnO 2 has already exhibited its performance as the primary electrode material for supercapacitors by providing a high capacitance. 21 It can be used with MOFs to develop a synergistic effect and improve active sites to enhance the overall electrochemical performance. For instance, Huang, Pang, and Lai et al. reported a simple in situ self-transformation method to obtain a MnO 2 @MOF composite material for adaptable energy storage devices. A solid-state flexible SC was fabricated based on prepared MnO 2 @MOFs and activated carbon as positive and negative electrodes, respectively, which exhibited an areal capacitance of 175 mF cm −2 at 0.5 mA cm −2 and possessed a maximum volumetric energy density of 5.1 mW h cm −3 (210 W h kg −1 ). 22 Liu et al. synthesized a pinecone-like core−shell composite with vertically grown MnO 2 nanosheet arrays decorated on the MFe 2 O 3 derived from Fe-MOF. A HSC device was assembled using pinecone-like MFe 2 O 3 @MnO 2 as the anode and urchin-like NiCo 2 O 4 as the cathode, and it showed a notable specific capacitance of 804.1 W kg −1 in the potential range of 1.1−0.3 V with an energy density of 86.8 W h kg −1 . The cyclic stability of the device was improved when the MnO 2 quantity is adjusted to the optimum level. 23 Pang and Zhu et al. synthesized Ni-HHTP MOFs in situ grown on the surface of MnO 2 . The composite material considerably increased the conductivity and supplied ample ion transport routes. Additionally, the assembled aqueous asymmetric supercapacitor (ASC) with MnO 2 @Ni-HHTP as the anode and activated carbon as the cathode showed a specific capacitance of 368.2 F g −1 at 1 A g −1 with a high energy density of 35.8 W h kg −1 at a power density of 600 W kg −1 with a capacitance retention of 87.4 percent at 3000 W kg −1 . Even after 3000 cycles, the assembled device still displayed a high coulombic efficiency of 95.4% and a capacitance of 233.1 F g −1 . 24 Huang and Shi et al. reported novel electrochromic MOF-based hierarchical selfassembled nanosheets (EV-HNSs) as the negative electrode for supercapacitors which are synthesized using negative electroactive organic viologen ligands and europium ions via a dualtemplate-directed approach. The obtained self-assembled nanosheets (EV-HNSs) showed an areal capacity of 186.25 mF cm −2 at 1 mA cm −2 in a potential window of −0.9 to −0.1 V (vs Ag/ AgCl). Further, an ASC device was fabricated using electrodeposited MnO 2 as the positive electrode, and EV-HNSs as the negative electrode. The ASC exhibited a high areal energy density of 9.4 μW h cm −2 at 775 μW cm −2 in an operating potential of 1.55 V. 25 Chen, Xu, and Jiao et al. synthesized a porous NiO-incorporated Ni-MOF/NF with a cylindrical cagelike structure to promote the transport of electrons and ions in the electrochemical energy storage process. The prepared NiO@Ni-MOF/NF delivered a high specific capacity of 1853 C cm −2 at 1 mA cm −2 . An HSC was constructed using NiO@Ni-MOF/NF as the anode and CNT as the cathode, respectively, which delivered a specific capacitance of 144 F g −1 at 1 Ag −1 with an energy density of 39.2 W h kg −1 at a power density of 7000 W kg −1 . It also displayed good cycling stability with 94% capacity retention after 3000 cycles. 26 Yang and Zhao et al. prepared core−shell MnO 2 nanotubes@nickel−cobalt−zinc hydroxide (NiCoZn−OH) nanosheets using MOFs as a template. The prepared MnO 2 @NiCoZn−OH electrode showed a specific capacitance of 1569.1 F g −1 at 1 A g −1 and a high rate performance of 54% retention at 30 A g −1 . An ASC device fabricated using MnO 2 @NiCoZn−OH and AC as positive and negative electrodes, respectively, delivered a superior capacitance of 130.7 F g −1 at 1 A g −1 with a high energy density of 49.4 W h kg −1 at 842.7 W kg −1 , and an excellent capacitance retention of 91.3% after 10,000 cycles. 27 In this work, we present a composite made of Zn-BDC and MnO 2 (MnO 2 @Zn-MOF) serving as the electrode material for the supercapacitor. First, we use a hydrothermal approach to synthesize α-MnO 2 nanoparticles. This synthesized α-MnO 2 is then in situ incorporated into the zinc-terephthalate MOF (Zn-BDC) using a simple solution phase synthesis technique. Here, α-MnO 2 is selected owing to its superior electrocatalytic ability compared with β-MnO 2 , ϵ-MnO 2 , and γ-MnO 2 phases reported in the previous literature study. 28 The MnO 2 @Zn-MOF nanostructure as electrode materials displayed a specific capacitance of 880.58 F g −1 at 5 A g −1 and a life cycling performance of 93% retention after 10,000 cycles at a current density of 10 A g −1 with a potential range of 0−0.6 V. The composite electrode material demonstrated a significantly improved performance due to its thin rod-like morphology with enhanced reactive sites and redox-active manganese oxide traces. The ASC device constructed using the as-prepared material as the anode and commercial CB as the cathode delivered a specific capacitance of 160 F g −1 at 3 A g −1 with a high energy density of 40.68 W h kg −1 at a power density of 2024 W kg −1 . The device also retained 90% of the initial capacitance after 10,000 cycles at 9 A g −1 .

Synthesis of α-MnO 2 Nanoflowers.
The α-MnO 2 nanoflowers were synthesized via a typical hydrothermal method. In this preparation, 1.5 mmol KMnO 4 and 5 mmol concentrated HCl were added to 15 mL of deionized water. The mixture was stirred vigorously for several minutes to form a transparent purple solution and then it is transferred into a 50 mL Teflon-lined stainless-steel autoclave, and the autoclave was sealed and heated at 140°C in a hot air oven for 15 h to obtain the precipitate. The precipitates were cooled down to room temperature, collected by centrifugation, and washed several times with distilled water. The final product was obtained by drying at 60°C in a hot-air oven overnight.

Preparation of Zn-MOF.
In the typical synthesis, 1.5 g of terephthalic acid (BDC) (organic linker) and 4.1 g of zinc acetate dihydrate [Zn(CH 3 COO) 2 ·2H 2 O] (metal precursor) were simultaneously dissolved in a 150 mL DMF solution. Then, 3 mL of triethanolamine was gradually added into the mixture under continuous magnetic stirring at room temperature and the stirring was continued for 36 h. Finally, the brownish-white precipitates were obtained. After centrifuging, the sample was washed several times simultaneously with ethanol, distilled water, and DMF. The final products were dried at 60°C overnight in a hot-air oven and stored for further character-ization. The final product, Zn-MOF, is named ZM, for our convenience.

Incorporation of α-MnO 2 Nanoflowers into Zn-MOFs.
Twenty milligram of the as-synthesized α-MnO 2 was added to 40 mL DMF, and this suspension was gradually added to 150 mL DMF solution containing 4.1 g zinc acetate dihydrate [Zn(CH 3 COO) 2 ·2H 2 O] and 1.5 g of terephthalic acid (BDC) under steady magnetic stirring. The brownish precipitates were formed upon the dropwise addition of 3 mL triethanolamine in the above mixture under constant magnetic stirring at room temperature for 36 h. Further, the precipitates were collected by centrifugation and washed several times with ethanol, distilled water, and DMF. The final products were dried at 60°C overnight in a hot-air oven. Then, dried precipitates were stored for further characterization. The final product, MnO 2 @Zn-MOF, is named MZM, for our convenience. The same procedure was used to synthesize MnO 2 @Zn-MOF with different quantities of MnO 2 (10, 30, and 40 mg) and named M-10, M-30, and M-40, respectively, for our convenience.

Material Characterizations.
The crystal structure of the prepared electrode materials was examined by employing powder X-ray diffraction (XRD, Rigaku D/Max ultima3i) using Cu K-α radiation (=0.1542 nm). Field-emission scanning electron microscopy (FE-SEM, Zeiss JSM-7500F) and transmission electron microscopy (TEM, JOEL JEM 2100F) were used to image the morphological features of the produced samples. Energy-dispersive spectroscopy (EDS, Zeiss JSM-7500F) is used for chemical characterization/elemental analysis of materials. X-ray photoelectron spectroscopy (XPS, ULVAC-PHI 5000 Versa probe III) was used to examine the elemental composition and valence states of the materials.

Electrochemical
Measurements. An electrochemical workstation (BioLogic SP-150) was used to investigate electrochemical characteristics. The three-electrode configuration was used for all electrochemical experiments, with the prepared electrode serving as the working electrode, a Pt counter electrode serving as the cathode, Hg/HgO serving as the reference electrode, and an aqueous electrolyte solution containing 3 M KOH as the electrolyte. Cyclic voltammetry (CV) and galvanostatic charge−discharge (GCD) studies were performed at different scan rates (5 to 80 mV s −1 ) and current densities (5 to 30 A g −1 ), respectively. Electrochemical impedance spectroscopy (EIS) measurements were recorded with an applied frequency range of 0.01 Hz to 100 kHz.
To prepare the anode, the as-prepared material (MZM), CB, and polyvinylidene fluoride (PVDF) at a weight ratio of 7:2:1 were mixed with a N-methyl proline (NMP) solution to form a slurry. The slurry was gently coated on pre-cleaned Ni foam (NF) (1 cm 2 ) and dried in a hot-air oven at 50°C for 12 h.
The specific capacitance of electrode materials will be calculated by using the known values of current density I (A g −1 ), discharge time Δt (s), the mass of the active materials m (mg), and the potential window range ΔV (V) 29 where I is the current density (A g −1 ), Δt is the discharge time (s), m is the mass of active material (mg), and ΔV is a potential window (V).

Construction of ASC.
The electrodes were fabricated as specified in the three-electrode system with a disc-shaped NF with a 1 cm radius and it was used in a two-electrode configuration. The conventional method was used to create the PVA/KOH gel electrolyte. 30 The pre-made PVA/KOH hydrogel was applied to the Whatman filter paper during the ASC construction process. The two thin stainless-steel discs (current collector) of a Swagelok-type cell were then sandwiched with our sample MZM/NF and carbon-black/NF. BioLogic SP-150 equipment was then used to conduct electrochemical tests. Using CV, GCD, and EIS techniques, the electrochemical characteristics of ASC were investigated. The following relationships were used to compute the specific capacitance (C, F g −1 ), energy density (E D , W h kg −1 ), and power density (P D , W kg −1 ) of the ASC device. 31,32 where V is the operating potential (V), M is the total mass of the active materials (mg) and Δt is the discharge time (s).

RESULTS AND DISCUSSION
The XRD patterns of the MnO 2 , ZM, and MZM electrode materials are depicted in Figure 1.     ZnO clusters with a wurtzite structure. 36 In MZM electrode materials, the corresponding peaks of α-MnO 2 were presented in the original position indicating no phase change. The relative intensity of the peak at θ = 9.8 is very high for MZM when compared with the ZM electrode material, which indicates the high crystalline nature of the material. Here, the α-MnO 2 acted as an auxiliary reagent to mediate the synthesis process reducing the agglomeration as well as enriching the structural defects of the MOFs resulting in high crystallinity. 37−39 The relative intensity of α-MnO 2 is significantly lower than the intensity of MOFs, thus indicating the presence of the trace amounts of α-MnO 2 in the MZM electrode material. Also, the XRD patterns of different quantities of MnO 2 -incorporated Zn-MOF (shown in the Figure S5) indicate that increasing the amount of MnO 2 to a certain extent, i.e., 20 mg show MOFs with a sharp intense peak at θ = 9.8°and by increasing the quantity of MnO 2 (30 and 40 mg), the peak intensity was decreased. It can be inferred that 20 mg incorporation is the optimal quantity to obtain high crystallinity.
The surface morphologies of the prepared electrode materials were investigated by FE-SEM analysis. Figure 2a−d shows the FE-SEM images of α-MnO 2 , ZM, and MZM electrode materials, respectively. Figure 2a shows the flower-like morphology of the pure α-MnO 2 . The irregular flake-like morphology was exhibited by the ZM electrode material (Figure 2b). Besides, the MZM material exhibited an inhomogeneous thin rod-like morphology, as shown in Figure 2c,d. This change in the morphology and size of the MOF can be attributed to the addition of α-MnO 2 , which has already been reported in previously reported works in the literature. 17,38,40 This thin rod-like morphology with comparatively smaller particles may be helpful in providing more metalactive centers for electrochemical reactions. Moreover, a few flower-like structures can be seen in the FE-SEM of MZM, which is indicative of the presence of α-MnO 2 as this appears to be exactly the same as the SEM images of pure α-MnO 2 (inset Figure 2d). The EDX spectrum analysis of the MZM electrode material is shown in Figure 2e

Electrochemical Analysis of ZM and MZM Electrode Materials.
A three-electrode cell was used to study the electrochemical properties of electrode materials where the Pt wire, Hg/HgO, and the prepared electrode were used as the counter, reference, and working electrodes, respectively, and the electrochemical tests were conducted in 3 M KOH. Figure 5a shows the CV curves of ZM and MZM electrodes measured in the potential range of 0 to 0.6 V vs Hg/ HgO at a scan rate of 5 mV s −1 . The non-linear curve and the sharp redox peaks indicate the faradaic pseudocapacitive behavior. A pair of prominent redox peaks about 0.31 and 0.46 V (vs Hg/HgO) can be seen, which was indicative of the transition between Zn(II) and Zn(III). These redox processes enable a faradaic charge storage mechanism in ZM electrodes, and similar charge storage behavior has been seen in other metal-BDC crystals in reported works. 53,54 The intercalation and de-intercalation of the alkali metal ion K + from the electrolyte with the free ZnO clusters in the electrode surface raise this set of strong faradaic redox peaks. 55,56 The redox process is shown below 57,58 ZnO K e ZnOK + + + (5) The unique morphology of MOFs with structural defects and easily accessible metal centers is helpful in the fast diffusion of K + ions, facilitating a better interaction between the electrode and electrolyte. 59,60 The CV curve of the MZM electrode exhibits a relatively larger integral area and a greater current density when compared to the ZM electrode which indicates a higher specific capacitance. It can be noted that the addition of α-MnO 2 produced additional redox peaks around 0.35 and 0.49 V in the MZM electrode. This is due to the surface adsorption− desorption of K + ions by α-MnO 2 to store energy. This involves the conversion of Mn(III) to Mn(IV) and Mn(IV) to Mn(III) by a reversible surface redox reaction between Mn 4+ and Mn 3+ ions. 61 Figure 5b,c represents CV curves of ZM and MZM electrode materials, respectively, at various scan rates of 5, 10, 25, 50, and 100 mV s −1 within a potential range of 0−0.6 V. At higher scan rates, the integral area under the CV curve and current density increased, and the shape of the curve remained consistent, suggesting strong reversibility of electrode materials. 65 It can be seen that the contour of CV curves of the electrodes is preserved even at high scan rates (100 mV/s), which guaranteed the ultrafast rate kinetics of the working electrode's faradaic process. 55 Moreover, CV curves of pure α-MnO 2 show clearly visible redox peaks at a lower scan rate of 5 mV/s ( Figure S1a) and slightly change with higher scan rates ( Figure S1b), which indicates that the process of energy storage by α-MnO 2 was mainly associated with a redox mechanism and not the reaction between the Mn4 + ions and hydroxide in the electrolyte. 66  The GCD measurements were carried out at varied current densities (5 to 30 A g −1 ) in the potential window range of 0 to 0.6 V. The GCD curves of ZM and MZM electrodes at a constant current density of 5 A/g are shown in Figure 5d, and the GCD plot of α-MnO 2 is given in Figure S2c (see the Supporting Information). Due to redox activities that occur during the electrochemical charge and discharge processes, all the electrodes show a nonlinear-shaped GCD curve. 67 This is consistent with the CV results. Notably, the MZM electrode displays a longer discharge duration and a larger integral area beneath the GCD curve than the ZM electrode, indicating improved electrochemical performance. Figure 5e,f shows the GCD curves of ZM and MZM electrodes, respectively, at different current densities (5−30 A g −1 ). Non-linear discharge curves at all current densities with the presence of a voltage plateau confirm the faradaic pseudocapacitive characteristics of the electrodes. 68 A similar kind of discharge process can be seen in GCD curves of α-MnO 2 ( Figure S1d). This nature of nonlinearity is attributed to voltage-dependent charge-transfer redox reactions. 69 During the discharge, a sudden potential drop could be seen, which represents energy losses arising from the internal resistance of the setup. 70 Moreover, while comparing GCD curves of different quantities (10,20,30, and 40 mg) of MnO 2incorporated Zn-MOFs ( Figure S6b), it can be observed that the MZM electrode material demonstrated longer discharging time than the M-10 electrode material, which exhibited further longer discharge-time than M-30 and M-40 samples. This suggests that, in accordance with XRD results, optimal quantity of 20 mg of MnO 2 incorporation improved the crystallinity of MOFs, which resulted in better electrochemical performance. Figure 5g illustrates the plot between the calculated specific capacitances and current densities of ZM and MZM electrodes. Specific capacitance values of ZM and MZM electrodes were calculated using the GCD curves using eq 1. At a current density of 5 A g −1 , the determined specific capacitance values of the α-MnO 2 , ZM, and MZM electrodes were 541.69, 610.83, and 880.58 F g −1 , respectively. The specific capacitances of the MZM electrode were around 1.6 times greater than pure α-MnO 2 and 1.4 times greater than the ZM electrode. At current densities of 5, 10, 15, 20, and 30 A g −1 , computed specific capacitance values for the MZM electrode were 880.583, 732.166, 638.575, 508.66, and 445.00 F g −1 , respectively. It is clear that specific capacitances of the electrodes decreased with increasing current densities, indicating the insufficient involvement of the electrode material in electrochemical processes at higher current densities. The reasons for the improvement in the electrochemical performance of the MZM electrode include (i) increased number of reactive sites, (ii) enhanced electronic conductivity, and (iii) improved redox activity arising from the inclusion of α-MnO 2 . EIS profiles in the frequency range of 0.1 Hz to 1 MHz were used to determine the electrode's ion diffusion and electron-transfer properties. Figure 5h Figure 5i displays the cyclic stability of the MZM electrode obtained at a high current density of 10 A g −1 . The MZM electrode retained 94% of its initial capacitance over 10,000 GCD cycles with a coulombic efficiency of 99.4%, indicating the good stability of the electrode material. The XRD pattern and FESEM image of the MZM electrode material after cycling is shown in Figure S7a,b. From the XRD pattern ( Figure  S7a), it can be inferred that the characteristic peak (at θ = 9.8°) of MOFs is retained which demonstrates good reversibility. The significant reduction in the intensity of peaks may be due to the presence of CB and PVDF, which are added during electrode preparation. From the FESEM image ( Figure S7b), it can be seen that the MOF almost maintains the same morphology after long cycles without any major changes. However, some of the particles seem to be aggregated due to the presence of PVDF and  CB. The comparison of specific capacitance of our electrode material with previously reported material is tabulated (Table  1).

ASC Device.
In order to examine the practical performance of the prepared electrode, a non-aqueous ASC was made by employing the as-prepared MZM electrode and commercial CB as the positive and negative electrodes, respectively, in the PVA + KOH gel electrolyte, as shown in the schematic diagram in Figure 6a. The PVA + KOH gel electrolyte was made by mixing 5 g of KOH and 10 g of PVA in 150 mL of distilled water and stirring it at 90°C continuously until a clear solution is formed, then it is dried at room temperature. Moreover, designing the ASCs requires careful consideration of the appropriate operating potential window (OPW) of the positive and negative electrodes. 80,81 Consequently, the CV experiment was conducted at a scan rate of 5 mV s −1 in the three-electrode setup to assess the OPW of the positive and negative electrodes, as shown in Figure 6b. The determined OPW was 0 to 0.6 V for the MZM electrode and −0.75 to 0 V for the CB electrode. Therefore, in order to attain higher energy and power densities, the device should operate at a potential of 1.35 V. The CV curves for the assembled ASC measured in various OPWs at a fixed scan rate of 5 mV s −1 are shown in Figure 6c. As a result of combined electric-double layer capacitance of CB and redox-faradic behavior of MZM, all CV curves exhibit a quasi-rectangular shape when the OPW range was altered from 0.65 to 1.35 V. 82,83 The CV curves of ASC in the potential range of 0−1.35 V at varied scan rates between 5 and 200 mV s −1 are shown in Figure 6d. Even at an increased scanning rate of 200 mV s −1 , the CV curves maintained their shape, indicating good rate performance. The GCD measurement was carried out in various OPWs at a current density of 5 A g −1 in order to further corroborate the OPW of ASCs (Figure 6e). The sloping discharge pseudo-plateau is visible on all GCD curves, and this observation is in accordance with CV results. Additionally, the ASCs could operate up to 1.35 V without any alterations in GCD plots. Figure 6f shows the GCD plots of ASC at different current densities ranging from 3 to 9 A g −1 . Specific capacitance values of the ASC at 3, 5, 7, and 9 A g −1 calculated using eq 1 were 160.72, 138.57, 113.17, and 84.37 F g −1 , respectively. To compare the change in specific capacitance at different current densities, the values are represented as a bar graph in Figure S4 (Supporting Information). Energy and power densities are also important in determining the supercapacitor's performance, which are calculated using eqs 2−4. A Ragone plot showing energy densities at various power densities of ASC is displayed in Figure  6g. At the lowest power density of 2024.98 W kg −1 , the ASC displayed the highest energy density value of 40.64 W h kg −1 . The EIS measurements were shown in Figure 6h as a Nyquist plot fitted with an equivalent circuit model, with an inset showing a closer view of lower values. The obtained solution resistance (R s ), charge-transfer resistance (R ct ), and Warburg impedance (W) values were 0.568, 0.519, and 2.477 Ω, respectively. This confirms that the device has a considerably low resistance thus promoting ion transfer for better conductivity and enhancing the overall performance of the device. Further, the ASC demonstrated long-term cyclic stability during 10,000 charge−discharge cycles at a constant current density of 9 A g −1 and maintained 90% of its original capacitance with a coulombic efficiency of 98% (Figure 6i). The comparison of energy density and power density of assembled ASCs with other devices from the literature is shown in Table.2.
The following reasons demonstrate the MZM||CB ASC's enhanced electrochemical performance: (i) inclusion of α-MnO 2 considerably improved the electrical conductivity, redox activity, and charge storage characteristics, (ii) commercial CB electrodes with higher electrical conductivity and chemical stability, (iii) MOF electrode material with unique morphology which binds to the electrode using CB facilitate quick ion movements and also offer a lot of active sites for electrochemical reactions, and (iv) large operational potential window of 0 to 1.35 V, which is equivalent of individual CB and MZM electrodes, which results in the high energy density.

CONCLUSIONS
In summary, α-MnO 2 nanoflower-incorporated zinc-terephthalate MOF (MZM) was synthesized by the solution phase synthesis technique. The prepared electrode material exhibited a specific capacitance of 880.58 F g −1 at 5 A g −1 with a 94% capacitance retention after 10,000 cycles at 10 A g −1 . The material showed superior electrochemical performance with enhanced conductivity, improved redox activity, and an increased number of reactive sites owing to the unique structure of MOFs and the addition of α-MnO 2. Moreover, the ASC assembled using MZM (anode) and commercial CB (cathode) electrodes displayed a high energy density of 40.68 W h kg −1 at a power density of 2024.98 W kg −1 and shows a specific capacitance of 160 F g −1 at 3 A g −1 . The device also demonstrated a good cycle stability of 90% retention of its initial capacitance after 10,000 cycles. Thus, α-MnO 2 nanoflower-incorporated zinc-terephthalate electrode material is considered an attractive option with great potential for the future generation of energy storage devices.
High-resolution XPS spectrum, CV curves, Nyquist plots, specific capacitance versus current density plot of ASC, powder XRD patterns, comparative CV curves and GCD curves, XRD pattern and FE-SEM image, CV curves of CB electrode at different scan rates (5,10,25,50, and 100 mV s −1 ), GCD curves of CB electrode at different current densities (5, 10, 15, 20, and 30 A g −1 ), and Nyquist plots of the CB electrode (PDF) The authors declare no competing financial interest.